Throttling valves are used in a wide number of process control system applications to control some parameter of a process fluid. While the process control system uses a throttling valve to control the pressure, level, pH or other desired parameter of a fluid, the throttling valve ultimately controls the rate of fluid flow.
Typically, a throttling valve includes a fluid inlet passageway coupled through an orifice to a fluid outlet passageway and a closure member disposed in the orifice which controls the amount of fluid flow therethrough. The closure member may include a valve plug having a surface which seats against a seat ring disposed at the orifice. During operation, the control system moves the valve plug towards and away from a surface of the seat ring to provide a desired fluid flow through the orifice and, therefore, the throttling valve.
The flow rate of a throttling valve is generally considered, by definition, to be a steady-state value for a given valve plug position and given pressure conditions. As a result, system designers have traditionally treated the flow rate of a throttling valve as a constant when designing a control system. In reality, however, the flow rate of a throttling valve does not remain perfectly constant but fluctuates during use. Changes in the flow rate of a valve may manifest themselves as momentary jumps in the valve flow rate or as persistent changes in the valve flow rate resulting in, for example, bi-stable flow rates. Although it has been surmised that the change in valve flow rate is related to turbulence, the precise cause of changes in the flow rate of valves has been, heretofore, unknown.
It is, however, generally known that turbulence is produced in a valve under most conditions. Turbulence is an irregular condition of fluid flow in which the pressure, the velocity, etc. of a fluid vary chaotically. Turbulence exists over a range of physical dimensions and time scales, i.e., from physically large fluid motions to physically small fluid motions and from rapid random changes to slow random changes.
Furthermore, during operation of a valve, the fluid in the valve must speed up to pass through the flow restriction created by the valve plug and the orifice. It has been conjectured that turbulence and/or the higher speed flow of a fluid in a valve may lead to flow patterns within the valve that are not stable. These unstable flow patterns may cause flow rate disturbances.
Flow rate disturbances, which may occur to a greater or lesser amount at different fluid pressures and flow rates, appear within a throttling valve over a wide range of frequencies and operate to effect the fluid flow through the valve. Some of the flow disturbances within a throttling valve occur at frequencies which are much greater than the characteristic frequency of the process being controlled or much greater than the frequency range over which typical control process equipment can respond. These high frequency flow disturbances manifest themselves as noise and are effectively filtered out by the mechanical flow capacitance inherent in the process. As a result, these high frequency disturbances do not significantly effect the operation of the control system.
Furthermore, some disturbances within a throttling valve occur at frequencies that are much lower than the characteristic frequencies of the process and the process control equipment. The process control system can compensate for these low frequency disturbances during closed-loop operation because the control system is able to recognize the slowly changing values of fluid flow caused by these low frequency disturbances and adjust the throttling valve accordingly.
However, disturbances that appear at frequencies on the order of the characteristic frequencies associated with the process and the process control equipment, that is, intermediate frequency disturbances, cause significant problems in the process control system because the process controller is changing process control parameters to compensate for these disturbances at about the same rate that the disturbances are appearing and disappearing. The process controller, therefore, has a hard time keeping up with these intermediate frequency disturbances which, in turn, leads to poor controller performance.
Up until the present, no one really understood what was causing intermediate frequency disturbances and, therefore, how to best negate these disturbances in a process. Furthermore, there has been no widely applicable solution for reducing the intermediate frequency disturbance in a valve, such as a throttling valve.